In a healthy relationship:
There are lots of things you can do to be a better boyfriend or girlfriend. Most of it starts with treating your partner the way you would like to be treated.
Listen (Really Listen): This doesn't just mean being quiet when the other person is talking. You really need to listen to your partner. Listen for what he or she is actually saying. Think about the words and don't interrupt.
Tell the truth: No matter what the situation always tell your partner the truth. If you always tell the truth, your partner is never going to catch you in a lie. And, telling the truth, builds trust.
Don't try to fix everything: Often your partner just wants to tell you what they are feeling. They just want you to "be there for them." You don't have to fix everything. Just be supportive.
Do things your partner wants to do to show you appreciate them and what they want to do: Relationships are about both giving and receiving. That means doing things your partner wants to do as well as the things you want to do. And, it never hurts to ask, "What would you like to do?"
Put yourself in the other person's shoes: Every once in a while ask yourself, "How would I feel if I was in the other person's shoes?" Would I feel respected, trusted, liked and loved? This is a very important question to ask yourself when you're deciding what to do.
Be clear about what you say: Just as guys often get accused to not listening, girls are often accused of not being clear when they talk with their partners. Some girls say, "I shouldn't have to tell my partner what I want. If he/she cares about me they should just know." If you're not communicating clearly don't be surprised if you're not getting what you want.
Have a positive attitude: It's especially important to have a positive attitude when your partner does something different than you would like. Before you start being negative and correcting the other person, ask yourself—"How important is it?" Nobody likes a complainer.
Stick up for yourself: You'll be a better person if you are clear about what you want and don't want --- what you'll do and not do. In this way you are being a better friend to both your partner and yourself.
Don't gossip about your partner: Think before you tell your other friends private things about your partner. Everyone shares stories about their friends. But you may be crossing the line if you start sharing private that could embarrass your partner. If your partner isn't your friend maybe it's time to end the relationship.
Don't do things just to make him/her jealous: There's harmless flirting and then there's doing things to control your partner by showing them how ready you are to leave and find a new love interest.
Don't be a cheater: If you're in an exclusive relationship, be exclusive.